Recent advances in silicon single photon avalanche diodes and their applications
Massimo GhioniPolitecnico di Milano, Dipartimento di Elettronica e Informazione
M. Ghioni Pavia, April 3, 2007
2Outline
• Single photon counting: why, what and how
• SPAD device technology: origin and evolution
• Single element SPAD detectorsrecent advancescustom SPAD vs standard CMOS technologyapplication cases
• SPAD array detectorsapplication cases
• Conclusions
M. Ghioni Pavia, April 3, 2007
3Why single photon counting?
For ultimate sensitivity in optical signal measurement !
straight digital technique
overcomes limits of analog measurements (circuit noise)
photon timing with picosecond precision
measurement of ultrafast optical signals
by Time Correlated Single Photon Counting (TCSPC)
M. Ghioni Pavia, April 3, 2007
4Why high sensitivity?
• Low sample concentration
• Minute samples
• Short exposure time
• Photon losses (poor collection, absorption, etc.)
• Low excitation power
• Greater magnification
• Ultra-weak emission (Raman scattering etc.)
M. Ghioni Pavia, April 3, 2007
5Photon counting/timing applications
photoncount
QuantumInformationProcessing
Metrology
MedicalPhysics
Military
Space Applications
Electronics
Biotechnology
Meteorology
detector calibration
primaryradiometric
scales
quantum standards
lighting
displays
IR detectors
lidar
quantum cryptography
quantum computing
single photonsources
entertainment
robust imagingdevices
nuclear
radioactivity
medical / non interactiveimaging
remote sensing
night vision
security
single moleculedetection
medicalimaging\
bioluminescencequantum imaging
hyper-spectralimaging
neutrino/cherenkov/ dark matter detection
environmental monitoring chemical – bio agent detection
photoncounting
QuantumInformationProcessing
Metrology
MedicalPhysics
MilitarySpace
Applications
Electronics
Biotechnology
Meteorology
detector calibration
primaryradiometric
scales
quantum standards
lighting
displays
IR detectors
lidar
quantum cryptography
quantum computing
single photonsources
entertainment
robust imagingdevices
nuclear
radioactivity
medical / non interactiveimaging
remote sensing
night vision
security
single moleculedetection
medicalimaging\
bioluminescencequantum imaging
hyper-spectralimaging
neutrino/cherenkov/ dark matter detection
environmental monitoring chemical – bio agent detection
source: www.photoncount.com
M. Ghioni Pavia, April 3, 2007
6Available detectors
Vacuum TubePMT
Currently used in photon counting/timing applicationsLimited quantum efficiency
Solid State APD (ordinary Avalanche PhotoDiodes)
No single photon detection
Special CCD (EM-CCD, I-CCD)Photon counting possible only at low frame ratesLimited time resolution
SSPD (Superconducting Single Photon Detector)Limited active areaNeed to be operated at < 4 K
SPAD (Single Photon Avalanche Diode)Best suited for photon counting/timing applications
M. Ghioni Pavia, April 3, 2007
7SPAD: reverse I-V characteristic
VREV [V]VBD
No avalanche
Avalanche
I REV
[mA
]
M. Ghioni Pavia, April 3, 2007
8APD vs. SPADAPD SPAD
Avalanche
ON
Quenching
Reset
Avalanche PhotoDiode Single-Photon Avalanche Diode
• Bias: well ABOVE breakdown
• Geiger-mode: it’s a TRIGGER device!!
• Gain: meaningless !!
• Bias: slightly BELOW breakdown
• Linear-mode: it’s an AMPLIFIER
• Gain: limited < 1000
M. Ghioni Pavia, April 3, 2007
9for SPAD operation…
mandatory
• to avoid local Breakdown, i.e.
• edge breakdown guard-ring feature
• microplasmas uniform area, no precipitates etc.
butbut forfor goodgood SPAD performance.....SPAD performance.....
further requirements!!
M. Ghioni Pavia, April 3, 2007
10Earlier Diode Structures
Haitz’s planar diode (early 60’s)
p
+n oxidemetal
guard ring-n
metal
5 µm
5 µm
Avalanche physics investigation• operated at low voltage (a few tens of Volt)• limited power dissipation during the avalanche (a few hundred milliwatt)• fabricated in ordinary silicon wafer with a planar technology
R.Haitz, J.Appl.Phys. 35, 1370 (1964), J.Appl.Phys. 36, 3123 (1965)
M. Ghioni Pavia, April 3, 2007
11Earlier Diode Structures
RCA reach-through diode (circa 1970)
• operated at high voltage (a few hundred Volts)• high power dissipation during the avalanche (around ten watt)• proprietary non-planar technology on a ultra-pure high-resistivity silicon
wafers
R. McIntyre, H. Springings, P.Webb, RCA Engineer 15, 1970
M. Ghioni Pavia, April 3, 2007
12Haitz’s planar diode
• Deep diffused guard ring
causes the photon detection efficiency (PDE) to be non uniform in the active zone
PDE = QE x η- QE = quantum efficiency
- η = avalanche triggering probability
M. Ghioni Pavia, April 3, 2007
13Haitz’s planar diode
- Haitz’s structure has drawbacks in applications requiring high-resolution photon-timing
- Long diffusion tail- Multi-exponential tail makes deconvolution more difficult
G. Ripamonti and S. Cova, Solid State Electron. 28, 925 (1985)T.A.Louis et al, Rev.Sci.Instrum. 59, 1148 (1988).
M. Ghioni Pavia, April 3, 2007
14Epitaxial SPAD structure
10
10
10
10
10
5
4
3
2
1
0 1 3 42 5100
Time (ns)
Cou
nts
- Shorter tail duration
- p+ implantation for VBD control
- Fully isolated devices on wafer
- Guard Ring still employed non-uniform PDE, non-exponential tail
M.Ghioni, S.Cova, A.Lacaita, G.Ripamonti, Electron. Lett. 24, 1476 (1988)
M. Ghioni Pavia, April 3, 2007
15Double-epitaxial SPAD structure
10
10
10
10
10
5
4
3
2
1
0 1 3 42 5100
Time (ns)
Cou
nts
• Short diffusion tail with clean exponential shape• Active area defined by p+ implantation• No guard-ring (uniform PDE)
• Adjustable VBD and E-field
• SUITABLE for array fabrication
neutral p layer thickness wtail lifetime τ = w2 / π2Dn
A.Lacaita, M.Ghioni, S.Cova, Electron.Lett. 25, 841 (1989)
M. Ghioni Pavia, April 3, 2007
16Double-junction SPAD structure
FWHM = 35ps
FW(1/1000)M = 214ps
FW(1/100)M = 125ps
FWHM = 35ps
FW(1/1000)M = 214ps
FW(1/100)M = 125ps
p-epi
hν +n
+
p++ p++
p
n-substrate
• Patterned p++ buried layer• No Tail (no carrier collection from neutral layer)• Suitable for small area devices (Φ ~ 10µm)
A.Spinelli, M.Ghioni, S.Cova and L.M.Davis, IEEE J. Quantum Electron. QE-34, 817 (1998)
M. Ghioni Pavia, April 3, 2007
17Device technology: prospect
• Two different approaches
standard CMOS technology
custom SPAD technology
have to face most requested improvements:
higher photon detection efficiency (especially in the red region)
larger active area (~ 100 µm)
shorter diffusion tail
M. Ghioni Pavia, April 3, 2007
18Custom SPAD technology
• Full process flexibility makes it possible to address the mostdemanding requirements
n
p+p
p
hν +n
+
→ Top epi-layer thickess/doping adjusted to increase PDE
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
400 500 600 700 800 900 1000Wavelength (nm)
Phot
on D
etec
tion
Effic
ienc
y 10 V7 V5 V
Excess Bias Voltage
M. Ghioni Pavia, April 3, 2007
19Custom SPAD technology
n
p+p
p
hν +n
+
0 400 800 1200 1600 2000Time (ps)
Cou
nts
100
1
2
3
4
FWHM = 35 ps
FW1/100M = 370 ps
10
10
10
10
→ Bottom epi-layer thickess adjusted to achieve short diffusion tail
M. Ghioni Pavia, April 3, 2007
20Custom SPAD technology
n
p+p
p
hν +n
+heavy phosphorus diffusion
p/p+ segregation gettering
→ Specific designed gettering processes for removing transition metal impuritiesresponsible for:
- thermal carrier generation (dark count rate - DCR)- carrier trapping (afterpulsing effect)
M. Ghioni Pavia, April 3, 2007
21Dark Count Rate (primary noise)
• Thermally generated carriers trigger avalanche pulses• Shot noise, equivalent to dark current in PINs / APDs
Thermal Generation via GR centers Field-Enhanced Generation
M. Ghioni Pavia, April 3, 2007
22Field-enhanced generation
Coulombic well Dirac well
• Poole-frenkel effect
barrier height lowered
• Phonon-assisted tunneling
barrier width decreased
Phonon process is thermally activatedTunneling is temperature independentOverall temperature dependence is a function of electric field
M. Ghioni Pavia, April 3, 2007
23
0.1
1
10
100
1000
10000
-80 -60 -40 -20 0 20Temperature (°C)
Cou
nts
(c/s
)
SPAD with "standard" electric
SPAD with "engineered" electric field
Custom SPAD technology
n
p+p
p
hν +n
+
→ Electric field engineered to avoid band-to band tunnelingField-enhanced generation less intenseDCR strongly reduces with temperature
M. Ghioni Pavia, April 3, 2007
24Large area SPADs: dark count rate
Practical Exploitation of DCR vs TPeltier cooling to -20°C
is simple / cheap / rugged
reduces DCR by a factor 25 – 1000.1
1
10
100
1000
10000
100000
-50 -40 -30 -20 -10 0 10 20Temperature (°C)
Cou
nts
(c/s
)
200 µm
50 µm
100 µm
25
100
Dark Count Rate (DCR)• Avalanche pulses triggered by
thermally generated carriers• Equivalent to the dark current in
PINs and APDs
Typical performance @5V excess bias voltage
M. Ghioni Pavia, April 3, 2007
25Large area SPADs: afterpulsing
Afterpulsing Effect• Carriers trapped during
avalanche• Carriers released later trigger the
avalanche• Increases noise and affects
correlation measurements
Characterization of afterpulsing• 200 µm detector
• 80ns deadtime• Time Correlated Carrier Counting
(TCCC) method
• Afterpulsing negligible after 1 µs
• Total afterpulsing probability:
~ 2% @ RT
~ 6% @ -25°C
M. Ghioni Pavia, April 3, 2007
26Large area SPADs: time response
By using a current pick-up circuit* and sensing the avalanche current at verylow level (< 100 µA):
FWHM not dependent on the detector diameter35ps FWHM checked for 200µm deviceat room temperatureVery stable response up to 4 Mc/s1
10
100
1000
10000
100000
11.5 12.0 12.5 13.0 13.5 14.0 14.5Time (ns)
Cou
nts
(c/s
)
FWHM = 35 ps
λλ = 820 = 820 nmnm
- clean exponential tail with 240 ps lifetime
* S.Cova, M.Ghioni, F.Zappa, US patent No. 6,384,663 B2, 2002
A.Gulinatti et al, Electron. Lett. 41, 272 (2005)
M. Ghioni Pavia, April 3, 2007
27Custom SPAD technology: pros & cons
PROs
• Flexibility: designer can modify process parameters & conditions
• Optimization of device structure can be pursued
• High-performance SPADs demonstrated with diameter up to 200 µm
• Progress of technology driven by detector requirements
CONs
• Monolithic integration of detector and electronics requires circuitcomponents specifically designed in the detector technology
• Dedicated silicon foundry is required
M. Ghioni Pavia, April 3, 2007
28CMOS based SPAD
• standard HV-CMOS technology• deep n-well to cut off the diffusion tail• p+n junction (intrinsically low PDE)
A. Rochas et al, Rev. Sci. Instrum. 74, 3263 (2003)
M. Ghioni Pavia, April 3, 2007
29CMOS-SPAD: experimental results
• low PDE @ 600-700 nm
• fairly high DCR @ Vexc>3V (φ = 12µm)
• DCR decreases slowly with T
PDE
F. Zappa et al, Optics Letters 30
DCR
, 1327 (2005)S.Tisa et al, IEEE-IEDM, 815 (2005)
0.8 µm HV-CMOS
M. Ghioni Pavia, April 3, 2007
30CMOS-SPAD: experimental results
Afterpulsing Time response
1E-06
1E-05
1E-04
1E-03
1E-02
0 5 10 15 20 25 30 35 40
Time (ns)
Afte
rpul
sing
Pro
babi
lity
Den
sity
(1/n
s)
55ns hold-off
• 2.6% total afterpulsing probability @ 55ns hold-off
• 35 ps time resolution FWHM
• long diffusion tail
F. Zappa et al, Optics Letters 30, 1327 (2005)
M. Ghioni Pavia, April 3, 2007
31CMOS-SPAD: pros & cons
PROs• Standard fabrication in silicon foundry, mature technology
• Straightforward integration: on-chip detector & electronics
• Small parasitic capacitance small avalanche charge for small detectorsbut NOT for wide devices (higher junction cap: 100 µm diam. CJ~ 1pF )
CONs
• High voltage CMOS process required
• No flexibility in processing
• SPAD’s with diameter > 50 µm not yet demonstrated
• Progress of technology driven by circuit requirements
M. Ghioni Pavia, April 3, 2007
32
Quenching circuits
M. Ghioni Pavia, April 3, 2007
33Quenching circuits
Passive quenching is simple...
… but suffers from
• not well defined deadtime
• τreset > 100 ns for (Cd + Cs) > 1 pF
• photon timing spread• et al
τreset=RL (Cd + Cs)
M. Ghioni Pavia, April 3, 2007
34Quenching circuits
Active quenching...
Output Pulses
P.Antognetti, S.Cova, A.LongoniIEEE Ispra Nucl.El.Symp. (1975)Euratom Publ. EUR 5370e
...provides::• short, well-defined deadtime• high counting rate > 1 Mc/s• good photon timing • standard logic output
M. Ghioni Pavia, April 3, 2007
35iAQC: integrated Active Quenching Circuit
F.Zappa, S.Cova, M.Ghioni, US patent 6,541,752 B2, 2003 (prior. March 9, 2000)F.Zappa et al., IEEE J. of Solid State Circuits 38, 1298 (2003)
Practical advantages
• Miniaturization mini-module detectors• Low-Power Consumption portable modules• Rugged and Reliable
Plus improved performance
• Reduced Capacitance• Improved Photon Timing• Reduced Avalanche Charge• Reduced Afterpulsing• Reduced Photoemission reduced crosstalk
in arrays
M. Ghioni Pavia, April 3, 2007
36Signal pick-up for improved photon-timing
• Avalanche current sensingat very low level (< 100 µA)
• Can be added to any existing AQC
S.Cova, M.Ghioni, F.Zappa, US patent No. 6,384,663 B2, 2002 (prior. March 9, 2000)
A.Gulinatti et al., Electron. Lett. 41, 20047445 (2005)
0 40 80 120 160 200Threshold voltage (mV)
25
75
125
0
50
100
150
Tim
e re
solu
tion
FWH
M (p
s)
50 µm activearea diameter
M. Ghioni Pavia, April 3, 2007
37Improved i-AQC with on-chip current pick-up and timing circuit
A. Gallivanoni, I. Rech, D. Resnati, M. Ghioni, and S. Cova, Optics Express 14, 5021 (2006)
M. Ghioni Pavia, April 3, 2007
38
Single element SPAD: application cases
Single molecule fluorescence spectroscopy
Fluorescence Lifetime Imaging (FLIM)
M. Ghioni Pavia, April 3, 2007
39Single molecule fluorescence spectroscopy
Fre-FAD complex
• Conformational dynamics of of biomolecules is crucial to their biological functions
• Electron transfer used as a probe for angstrom-scale structural changes
• Measure fluorescence lifetimes (down to < 100ps) to gauge conformational dynamics
H. Yang, G. Luo, P. Karnchanaphanurach, T.M. Louie, I. Rech, S.Cova, L. Xun, and X. Sunney Xie, Science, 302(5643), 2003
M. Ghioni Pavia, April 3, 2007
40Single molecule fluorescence spectroscopy
• Correlation analysis revealed conformational fluctuation at multiple time scales spanning from hundreds of microsecond to seconds
Yang, H., et al., Science, 302(5643), 2003
M. Ghioni Pavia, April 3, 2007
41Single Photon Timing Module SPTM
• Compact (82x60x30mm)• Single power supply (+15V)• Controlled Temperature
(Peltier cell)• Software controlled settings• On-board fast counters• RS-232 data transmission• Time-resolution: 60ps • Dark Counts: down to 5 c/s• PDE: 45% @ 500nm
• I.Rech et al., IEEE J. of Sel. Topics in Quantum Electronics, vol.10, 788 (2004)
M. Ghioni Pavia, April 3, 2007
42SPTM performance in the Harvard set-up
Instrument Response Function (IRF)
with SPTM and with PerkinElmer SPCM
• Time-resolution: 60ps• Dark Counts: down to 5 c/s• Quantum Efficiency: 45% @ 500nm
• I.Rech et al., IEEE J. of Sel. Topics in Quantum Electronics, vol.10, 788 (2004)
M. Ghioni Pavia, April 3, 2007
43Fluorescence Lifetime Imaging (FLIM)
FLIM image of the autofluorescence of daisy pollen grains• 64 µm x 64 µm area (256 pixels/axis)• 0.6 ms/pixel acquisition time → 2 min total measurement time
Courtesy of Picoquant GmbH, Germany
M. Ghioni Pavia, April 3, 2007
44
SPAD arrays
M. Ghioni Pavia, April 3, 2007
45SPAD arrays
Photon Counting inAdaptive optics in astronomyParallel Fluorescence Correlation SpectroscopyMultiphoton multifocal microscopyChemiluminescent assay analysis
Photon Timing in
Fluorescence lifetime imaging
Basic goals - increase throughput- miniaturization, lower system cost
Two approaches- Dense CMOS-based SPAD arrays
3D imaging
- SPAD arrays with limited pixel number (< 100) and large pixel area
M. Ghioni Pavia, April 3, 2007
46SPAD arrays and optical crosstalk
Origin: hot-carrier luminescence 105 avalanche carriers 1 photon emitted
A. Lacaita et al, IEEE TED (1993)
Approach:• Optical isolation between pixels• Avalanche charge minimization
M. Ghioni Pavia, April 3, 2007
47
SPAD arrays: application cases
Tip-tilt and curvature sensors for adaptive optics
Large element SPAD array for protein microarray detection
M. Ghioni Pavia, April 3, 2007
48
STRAP = System for Tip-tilt Removal with Avalanche Photodiodes
STRAP Adaptive-Optics System of the VLT Observatory (Chile)European Southern Observatory - ESO
D.Bonaccini et al, Proc. SPIE Vol. 3126, p. 580-588, Adaptive Optics and Applications; R.K.Tyson, R.Q.Fugate Eds., 1997
Adaptive Optics
M. Ghioni Pavia, April 3, 2007
49Hybrid four-quadrant SPAD module
2x2 lenslet array
Peltier
Spacer Ceramic
Centering Ceramic
Quenching, protection circuit and other electronics developed by Polimi and Microgate
4 SPAD chips supplied by PerkinElemerCourtesy of A. Silber (ESO)
M. Ghioni Pavia, April 3, 2007
50
100µm, 80µm, 50µm pixel diameter
Replace the single SPAD chips in STRAP modules
Monolithic four-quadrant SPAD detector
M. Ghioni Pavia, April 3, 2007
51SPAD-Array (SPADA)
60 element array with circular geometry
Fully parallel – 20 kfps
4 sets of pixels
- Curvature sensor for AO systems
F. Zappa et al, IEEE PTL 17, 657 (2005)
M. Ghioni Pavia, April 3, 2007
52SPADA detector head
M. Ghioni Pavia, April 3, 2007
536x8 SPAD array detector
Chemiluminescent protein microarray for “in-vitro” allergy diagnosis
50 µm pixel diameter
240 µm pitch
M. Ghioni Pavia, April 3, 2007
542-D photon counting module: optics
• NA = 0.3
• FOV = 2,064 mm
• η ~ 8%
• Magnification 1:1
Ottica di raccolta Ottica di focalizzazione
Filtri ottici
Microarray SPADA
Collectingoptics
Focusingoptics
Optical filters
M. Ghioni Pavia, April 3, 2007
552-D photon counting module: mechanics
Filter holder
20cm20cm
8.5cm8.5cm17cm17cm
Slide tray
X Y
θ
M. Ghioni Pavia, April 3, 2007
56Conclusion
SPADs in planar silicon technology offer high performance at low-cost
HV-CMOS industrial technologies produce remarkable devices: Single SPAD’s (< 50µm diam); SPAD Arrays (<10% FF), Integrated PC-Systems
Custom CMOS-compatible technologies provide today’s top-performance SPAD’sand flexibility to sustain continuing evolution and progress
Monolithic iAQCs open the way to miniaturized modules (down to the chip scale)
Remarkable results obtained in diversified applications: DNA and Protein Analysis; Single-Molecule Spectroscopy; Wavefront Sensors in Adaptive Optics; etc.
Results of decades of research made widely available by a new spinoff company
www.microphotondevices.com